"Gravity studies of the structure of the Vatnaöldur and Veiðivötn crater rows, South Central Iceland"
Jeanne M. Giniaux - 27. June at 15:00, Askja, room 131
Magnús Tumi Guðmundsson, professor and Guðrún Larsen
Gylfi Páll Hersir from Íslenskum orkurannsóknum (ÍSOR)
A gravity survey was carried out to study the structure and geometry of craters and underlying diatremes formed in two major explosive basaltic fissure eruptions, the ~60 km long AD 871±2 Vatnaöldur and the AD ~1477 Veiðivötn fissures in South Central Iceland, both belonging to the Bárðarbunga volcanic system. A minor effusive phase at the end of the Veiðivötn eruption filled the bottom of the large phreatomagmatic craters from the main explosive phase, while the Vatnaöldur eruption was almost purely explosive. Six gravity profiles were surveyed across six explosive craters, using a LaCoste&Romberg gravimeter and kinematic GPS. Complete Bouguer anomalies were obtained by integrating the gravitational effects of the mass of the topography using a high-resolution DEM. Studies of the subsurface geometry and structure were made using 2.5-D gravity forward gravity models. The results indicate 100-300 m deep diatreme structures below the Vatnaöldur and Veiðivötn craters. The purely explosive edifices at Vatnaöldur Innri are filled with relative low-density unconsolidated water-saturated tephra, and have much narrower diatremes than those seen in the Veiðivötn fissure, which experienced a late effusive stage leading to lava-filling of the craters. It is proposed that softer subsurface formations at Veiðivötn lead to wider diatremes than found at Vatnaöldur. At least at one of the craters in Vatnaöldur, the activity may have lasted long enough to lead to minor effusive activity, as a relatively high density body, assumed to be lava, fills the diatreme.